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Opinions on 3.90/3.91 non LSD diff for M42


tashakes

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After much searching, I have decided that I need to change my 3.64 diff for a 3.90 or 3.91 for my M42 5 spd swap. I don't care for LSD because I am not tracking nor are my skills that well developed that I think it would make a difference on street driving. A 49 state 76 or a 5 speed e21 are my targets. Prefer the 76 so I don't have to deal with half shafts and spacers.

What I am looking for is quicker low rev acceleration. My experience with the swap is that the nature of the M42 is you get the power on the upper side of the power band. So basically I need to get the revs higher in order for me to get the car moving quicker from a stand still. From what I have read, the 3.90/3.91 diff will make a difference when pulling off a stoplight, but will make the revs higher at highway speeds

Am I talking stupid, or are my theories correct? Is this is good idea? Good compromise? I don't do a lot of highway driving.

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Swapping '02 diffs is an easy bolt-in operation, particularly if you use a '76 3.90, and non-LSD diffs are so inexpensive, I don't see a lot of downside here!

I've lived with my '76 3.90, and the stock 4-speed, for 39 years and 105,000 miles and only became "aware" of the extra engine noise in the last 5 years, as my daily drivers got quieter and quieter, and as their highway cruising revs went lower and lower. But that's with a 4-speed. With an overdrive 5-speed, that issue goes away.

BMW installed the 3.90 -- or at least this was what they said at the time -- to recover some of the around-town "OOMPH" that was lost due to increased emissions controls. And, although I have absolutely nothing to show it, we believed -- way back when -- that the 3.90 reduced the 0-60 times by a half second over the 3.64. So, expect some change, but don't expect a different car!

Regards,

Steve

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An E30 318is with that same engine has a 4.11 final drive (convertibles have a 4.27) which is a good match for the M42 re acceleration without beating you to death on the highway.  I've discovered that my 318is is turning the same revs at 70 as my 2002 with a 3.64 diff and an overdrive 5 speed. 

 

Given that the 2002 is several hundred lbs lighter than an E30, a 3.90 would give you good acceleration plus unobtrusive revs at highway speeds.  Or for more accelerative oomph without straining the engine, find a 4.11 from a 1600.

 

mike

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The 4.11 fitted to 1600s wasn't too tall for the M10 engine--gave you significantly more revs at highway speeds than a 3.64, but as I'm sure you know, the M42 motor is a lot happier (and quieter) at 4k rpm than an M10.  And you'd have some pretty impressive acceleration with a 4.11 considering the weight advantage of an '02 over an E30.  

 

I'll bet there are plenty of serviceable 4.11 short nose diffs lurking around--those diffs are pretty long lived, just like the 3.64s and 3.90s...

 

But if you're gonna do a lot of highway driving, a 3.90 or even a 3.64 will be a better choice.  

 

mike

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Yes, early 1600-2s and very early 2002s got long-nose differentials. Those are the difficult-to-service models due to the non-availability of parts. Then both switched over to short-nose differentials, the one's most of us know and which continued on the e21 cars. You want a short-nose. But don't worry a lot about this issue: long-nose differentials are QUITE RARE whereas short-noses are VERY COMMON! The switchover was in '68! It is highly unlikely you will ever SEE a long-nose differential!

Regards,

Steve

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